1. Field of the Invention
The invention is directed to an active beam catcher, in particular for a fanned laser beam of a line laser transmitter.
2. Description of the Prior Art
Positioning devices which emit laser beams for marking are widely used for positioning tasks in the building industry. Particularly, at large distances and with poorly reflecting substrates and strict requirements for absolute positioning accuracy, beam catchers that are associated with the positioning devices, are used for reliably finding the beam, for exact marking and for definitive determination of a positional deviation. In line laser transmitters, a line, instead of a punctiform light spot, is projected on the wall by a laser beam that is fanned out in an angular sector. In the least favorable case, the tilt error in the projection direction, the tilt offset perpendicular to the projection direction and the straightness error of the fanned out laser beam add up to a total error in positioning accuracy which, moreover, increases linearly with an increasing distance from the positioning device. This is particularly significant because positioning with modern positioning devices and beam catchers is reliable over large distances of greater than 100 m even in unfavorable, e.g., bright, surroundings. In favorable, e.g., dark, surroundings, however, it would still be possible to detect the laser beam over a multiple of the distance, although the total error would be too large. It is of great practical importance for the user to prevent positioning of this kind with an excessive total error.
In conventional passive beam catchers, optical devices such as splitters or Fresnel lenses are used to convert minimal positional differences in the impinging laser light beam into qualitatively different displays. In conventional active beam catchers, a photodetector that is supplied by a power source, serves to receive the laser light beam, and a display connected to an evaluating unit is used to determine the offset of the light beam relative to the reference point.
According to U.S. Pat. No. 3,894,230, an active beam catcher for a pulsating light beam has a photodetector with a plurality of photodiodes which are offset along a line in a defined manner with respect to a reference point in the form of a pulse-controlled photodiode array, an evaluating unit, and a numerical read-out display of the offset of the light beam with respect to the reference point.
According to European Publication EP0426287, an active beam catcher for a light beam has pairs of photodetectors so that the determined, displayed center position of the light beam can be determined independent from its light spot size, and the energy distribution in the beam cross-section. No decision is made with respect to a possible total error.
It is the object of the present invention to realize an active beam catcher with which possible positioning with an excessive total error is prevented when the beam catcher is used with an associated line laser transmitter.